This invention relates to a missile optical waveguide which is wound as a package body and can be withdrawn from the package body during flight of the missile.
It is known to use optical waveguides for guiding missiles. Image data and status data are transmitted in real time from the missile to the ground and, at the same time and in the opposite direction, guiding and switching commands are transmitted from the ground to the missile. The signals are transmitted over a distance of 100 km or more. The optical waveguide has a high transmission band width and low damping.
The optical waveguide comprises a glass fiber and a coating, usually made of acrylic, which surrounds the glass fiber for protecting it against moisture and mechanical damage. The waveguide has an overall diameter of approximately 250 .mu.m and is normally carried along by the missile in the form of a wound body stationary on the rearward end which is wound off it during flight in the axial direction of the wound body. In order to prevent the wound body from disintegrating into individual wound layers during the winding-off operation, the optical waveguide must be wound onto the wound body with a sufficiently high tension (and usually also with the aid of a contact adhesive) which, at the crossing points of superimposed wound layers, leads to local compressive tension peaks because of the point contact. In addition, during the winding-off, the glass fiber is subjected during the flight to considerable bending stress and tensile stress, particularly since the acrylic coating, because its modulus of elasticity is low in comparison to glass, does not contribute to the increase of the tensile strength. As a result of its additional mass, the coating reduces the breaking length of the optical waveguide. Therefore, in the case of increase withdrawal speeds, microfine surface cracks of the glass fiber or even a bending break or a complete tearing-off of the optical waveguide will frequently occur, and there will therefore be an interruption, or at least a considerable impairment of the optical signal transmission.
It is also known to reinforce the optical waveguide by an additional covering with tension threads made, for example, of kevlar which extend unidirectionally in the longitudinal direction of the optical waveguide (European Patent Document EP 01 92 570 A1). However, adding the kevlar coating increases the outside diameter of the optical waveguide to approximately twice the size, while the length-related mass quadruples. Adding a kevlar coating to an optical fiber having a relatively long length leads to a considerable increase in weight and volume and is suitable only to a limited extent for a protection against disturbing crack formations in the glass fiber.
It is an object of the invention to provide a missile optical waveguide for signal transmission which is constructed in a simple manner and which relieves tension in the glass fiber, even without enlargement of volume of the round body, thereby providing an effective protection against a crack formation that may endanger operation of the missile.
According to the invention, this and other objects are achieved by means of a missile optical waveguide having a rectangular outer contour and a non-uniform cross-sectional wall thickness with the greatest thickness being at the corners.
In the case of the missile optical wavelength according to the invention, the optical waveguide has a cross-sectional configuration that is optimal for the withdrawal from the wound package fixed to the missile. Specifically, the special distribution of the material of the outer layer on the thickened corner zones facilitates an advantageous axial winding-off operation, where the waveguide is twisted once per withdrawn package winding about the longitudinal axis, thereby providing an increased expansion of material at the corner zones which is significantly larger in comparison to a circular cross-section of an equal area. Thus, the outer coating bears tensile force and, as a result, the inner light transmission fiber is effectively relieved from overstraining which may lead to cracks. Also, a punctiform pressure load of the optical waveguide in the wound package and the thus induced increased damping of the optical transmission is prevented by the mutual surface contact of superimposed wound layers. The individual package windings under the effect of the winding tension cannot be pressed into the grooves of the wound layer disposed beneath it and end up clamped therein.
The optical waveguide constructed according to the invention therefore ensures, in a surprisingly elegant manner, a uniformly broad-band signal transmission with a very low damping, even in the case of high flying speeds and long transmission paths between the missile and the ground station.
For certain embodiments of the invention, the outer coating has a square outer contour and a circular inner contour corresponding to the cross-section of the glass fiber, thereby facilitating manufacture of the waveguide.
In certain embodiments of the invention, fiber reinforcements are embedded in the outer coating which are expediently arranged in the corner areas of the outer coating whereby, while a small cross-sectional surface of the outer coating is maintained, a further relief of the glass fiber with respect to the tensile force is achieved. The fiber reinforcements, which are locally restricted to the corner areas, in view of a design of the outer coating that is highly appropriate for the load, extend preferably unidirectionally in the longitudinal direction of the optical waveguide and have a higher modulus of elasticity than the light transmitting fiber. In certain embodiments of the invention, the reinforcement fibers consist preferably of kevlar fibers and/or carbon fibers.
In certain embodiments of the invention, the package body is arranged in a non-rotatable manner, and the optical waveguide is withdrawn during the flight in the axial direction of the package body. The rectangular cross-sectional geometry of the outer coating provides the individual optical waveguide windings of the package body with sufficiently large mutual contact surfaces for an adhesive gluing preferred for the stabilization of the package body.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.